The present invention relates to a vehicle brake system incorporating an anti-locking device for the wheels and a retarder with monitored actuation, of the type comprising first means prohibiting the actuation of the retarder at least from the moment a regulation request is sent to said anti-locking device of the wheels.
It should first be reminded that retarders are devices adapted for slowing down the vehicle before and during the effective action of the brakes, via a manual or foot/pedal control actuated by the vehicle driver. The electromagnetic retarders, commonly in use, comprise a number n of independent switching ways, each one of them providing part of the deceleration torque. Such a type of retarder can be actuated or controlled via gradually preadjusted pressure switches (hydraulic or pneumatic ones), inserted in the braking circuit, or via switches integrated or not in the brake pedal block, successively acting depending on the depth of actuation or down travel of said pedal and thus providing the control or actuation, position by position, and under certain enabling conditions, of the n positions of the retarder operation. Provision can also be made for the switching of one or several ways of the retarder for each triggering of the pressure switch or cutout switch.
This being said, the purpose of the first aforesaid means is of course to prohibit the operation of the retarder when the anti-locking device of the wheels detects a skidding condition deemed as critical. As a matter of fact, if the retarder (with manual and/or foot actuation) would continue its action at that time, the risk of skidding of the wheels would be increased, thereby interfering with the action of said device.
For the anti-locking devices to be able to perform this known monitoring function of the retarder, they are provided with a so-called "regulation" terminal from which the desired piece of information can be picked up.
These devices also incorporate a so-called "alarm" terminal which controls a failure signalling light located on the dashboard of the vehicle, and from which the retarder can also be monitored and its actuation inhibited in case of failure of the anti-locking device of the wheels. It can be understood, in fact, that if this device has not operated or if it operates in a defective way, the operation of the retarder must be prevented in order not to further increase the risk of skidding of the vehicle wheels.
It is known still, in the brake systems of the type under consideration, to used a speed information, originating for example from an electronical tachograph or from a minitachogenerator attached to the gearbox in order to inhibit the actuation of the retarder when the vehicle is running at low speed (speed lower than a determined threshold) or at standstill. Since the electromagnetic retarder drains current, this avoids actuating it unnecessarily.
To sum it up, it can be seen that in the known brake systems--and as may also be the case in a brake system according to the present invention--the operation of the retarder may be prohibited:
in the case of a regulation request received by the anti-locking device of the wheels;
in the case of a failure of this device; and
when the vehicle is at standstill or running at low speed.
The present invention is concerned with the conditions required for re-enabling the actuation of the retarder subsequent to its prohibited operation, especially when it follows upon a regulation request received by the anti-locking device of the wheels.
In the systems known, this re-enabling function is obtained immediately following a request for regulation sent from the anti-locking device of the wheels, whereby the resetting into operation of the retarder, however, is enabled only gradually, position by position way by way or by groups of ways), with each time a controlled time-delay between two successive positions, this with a view to avoiding too rapid a resetting of the retarder.
The drawback of this known solution is that it results in having a greater instability of the anti-locking device of the wheels. Actually, the fact that the retarder may be effective again almost immediately upon the completion of the anti-locking regulation may bring about, under conditions of poor road grip or in the case of a slightly loaded vehicle, a skidding condition sufficient to trigger again said device before the end of the braking action. In other words, even when proper care is taken for a gradual and time-delayed resetting or restarting of the retarder by foot or manual control, the anti-locking device of the wheels may, under some circumstances connected with the coating of the road or with the vehicle, be disturbed by the restarting of the retarder during the braking action.